Could chemical weapons in Syria force Obama's hand?

Such measures might include establishing a no-fly zone over northern Syria, a step that already has bipartisan support in Congress, or creating "humanitarian corridors" for refugees to move along and for getting food and other supplies to the civilian population.

Probably a last resort, Mr. Korb says, would be "to send in special forces to grab the chemical weapons."

Pentagon officials said last month they were preparing a list of calibrated measures that the president could order in the event that chemical weapons are used. In response to the Israeli claims Tuesday, Pentagon spokesman George Little said the US "continues to assess reports of chemical weapons use in Syria."

Last week Britain and France sent a letter to United Nations Secretary-General Ban Ki-moon, in which they claim to have credible evidence that Syria had used chemical weapons more than once since December. The UN has assembled a team of experts that is awaiting permission from Assad to enter the country to investigate charges of chemical weapons use.

Calling any such use "entirely unacceptable," Mr. Little said, "We reiterate in the strongest possible terms the obligations of the Syrian regime to safeguard its chemical weapons stockpiles, and not to use or transfer such weapons to terrorist groups like Hezbollah."

The reference to Hezbollah, the radical Muslim organization in Lebanon known to be aiding Assad in his fight with the armed Syrian opposition, appeared to be a tip of the hat to Israel, which considers Assad's possible transfer of chemical weapons to enemies of Israel one of its greatest concerns.